1. Field of the Invention
The present invention relates to a liquid crystal display device of a multi-gap mode configured to set a thickness of a liquid crystal layer for each color of a color filter.
2. Description of the Prior Art
As compared to a cathode ray tube (CRT), a liquid crystal display device has advantages in that it is small in thickness and weight, and low in driving voltage and power consumption. For this reason, liquid crystal display devices are applied to various electronic devices including televisions, notebook personal computers (PCs), desktop PCs, personal digital assistants (PDAs), cellular telephones, and the like. In particular, an active matrix liquid crystal display device provided with thin film transistors (TFTs) as switching elements for respective picture elements (sub pixels) exerts excellent display characteristics almost equal to a CRT owing to high driving performances. Accordingly, active matrix liquid crystal display devices are now used in various fields, such as desktop PCs and televisions, where CRTs have been conventionally applied.
In general, a liquid crystal display device includes two substrates and liquid crystal which is sealed between these substrates. Picture element electrodes, TFTs, and the like are formed for respective picture elements on one of the substrates. Meanwhile, color filters opposed to the picture element electrodes, and a common electrode common to the respective picture elements are formed on the other substrate. In this specification, the substrate including formation of the picture element electrodes and the TFTs will be hereinafter referred to as a TFT substrate, and the substrate to be disposed opposite to the TFT substrate will be hereinafter referred to as a counter substrate. Moreover, a structure formed by sealing the liquid crystal between the TFT substrate and the counter substrate will be hereinafter referred to as a liquid crystal panel.
Conventionally, a twisted nematic (TN) liquid crystal display device, which is configured to seal horizontal alignment type liquid crystal (liquid crystal having positive dielectric constant anisotropy) between two substrates and to subject liquid crystal molecules to twisted alignment, has been widely used. However, the twisted nematic liquid crystal display device has a disadvantage of a poor view angle characteristic where contrast and color tone vary largely when a screen is viewed from an inclined angle. For this reason, a multi-domain vertical alignment (MVA) liquid crystal display device having a more favorable view angle characteristic than the TN liquid crystal display device has been developed and put into practical use. In the MVA liquid crystal display device, vertical alignment type liquid crystal (liquid crystal having negative dielectric constant anisotropy) is sealed between a TFT substrate and a counter substrate, and a plurality of regions, having mutually different alignment orientations of liquid crystal molecules upon application of a voltage are formed in one picture element.
Although the MVA liquid crystal display device has a more favorable display characteristic than the TN liquid crystal display device, the MVA liquid crystal display device still has a disadvantage of reduction in contrast when a screen is viewed from a certain direction (from obliquely above, for example). To solve this disadvantage, as disclosed in Japanese Patent Publications Nos. 3027805 and 3330574, and in Japanese Unexamined Patent Publication No. 2002-182036, a liquid crystal display device configured to dispose an optical compensation film (a retardation film) between a liquid crystal panel and a polarization plate has been developed and put into practical use.
FIG. 1 is a schematic diagram showing a configuration of a MVA liquid crystal display device disclosed in Japanese Patent Publication No. 3027805. As described above, a liquid crystal panel 10 is configured to seal vertical alignment type liquid crystal between a TFT substrate and a counter substrate. A polarization plate 11 is disposed on one of surfaces (which is on the lower side in FIG. 1) of this liquid crystal panel 10, and a polarization plate 12 is disposed on the other side (which is on the upper side in FIG. 1). Absorption axes of these polarization plates 11 and 12 are mutually orthogonalized.
Viewed from the liquid crystal panel 10 side, a retardation film 13 and a retardation film 14 are sequentially disposed between the liquid crystal panel 10 and the polarization plate 12. Assuming that refractive indices in the X direction, the Y direction (these are the in-plane directions), and the Z direction (the thickness direction) are Nx, Ny, and Nz, respectively, the retardation film 13 has positive refractive index anisotropy which satisfies Nx>Ny=Nz. On the contrary, assuming that refractive indices in the X direction, the Y direction, and the Z direction are Nx, Ny, and Nz, respectively, the retardation film 14 has negative refractive index anisotropy which satisfies Nx=Ny>Nz.
Incidentally, there has been disclosed a multi-gap liquid crystal display device, in which the thicknesses of a liquid crystal layer (cell gaps) in a red (R) picture element, a green (G) picture element, and a blue (B) picture element are adjusted by the thicknesses of color filters in order to correct differences in optical rotations of liquid crystal molecules which vary depending on optical wavelengths. In this type of the multi-gap liquid crystal display device, the thicknesses of the liquid crystal layer for the respective picture elements are set to satisfy the relation of the blue picture element <the green picture element the red picture element, for example. In this way, a color representation characteristic particularly in a multiple tone display is improved.
However, in the multi-gap liquid crystal display device, it is considered difficult to improve a view angle characteristic by use of the retardation films. The reason will be described below.
When a phase difference in the in-plane direction (the X direction or the Y direction) of the retardation film is Rin and a phase difference in the thickness direction (the Z direction) thereof is Rth, the values Rin and Rth will be defined by the following formulae (1) and (2):Rin=(Nx−Ny)×d  (1)Rth=((Nx+Ny)/2−Nz)×d  (2)
Here, d is the thickness of the retardation film. Optimal values of the phase difference Rin in the in-plane direction and the phase difference Rth in the thickness direction has a correlation with retardation possessed by the liquid crystal layer. The retardation RLC of the liquid crystal layer will be calculated by the following formula (3):RLC=Δn×dk  (3)
Here, Δn is a difference in the refractive indices of the liquid crystal layer between extraordinary ray and ordinary ray, and dk is the thickness of the liquid crystal layer.
However, as described previously, the thicknesses of the liquid crystal layer vary depending on the colors of the color filters in the multi-gap liquid crystal display device. Accordingly, the values of retardation of the liquid crystal layers vary among the red picture element, the green picture element, and the blue picture element. Therefore, optimal conditions of the retardation film also vary among the red picture element, the green picture element, and the blue picture element.
The retardation film is generally formed by stretching a polymeric film.
Accordingly, the phase difference Rin in the in-plane direction and the phase difference Rth in the thickness direction are uniform, and it is extremely difficult to partially change the phase difference Rin in the in-plane direction and the phase difference Rth in the thickness direction. Therefore, even when the phase difference Rin in the in-plane direction and the phase difference Rth in the thickness direction are for example set appropriately for the red picture element, the light will pass through in an oblique direction in the green picture element or the blue picture element. In this case, a portion supposed to be seen actually in black will be observed in green or blue in the green picture element or the blue picture element. In addition, the color in that portion may change depending on the viewing angle.